The phenomenon of ferromagnetic saturation plays an important role in the prior art high current transformer resistance meters. The main inductance L of the transformer drastically drops when the magnetic core is saturated. This effect facilitates it to measure the ohmic resistance after the inductive voltage drop has decayed to zero.
The resistance of Delta-connected windings in transformers is difficult to measure and it can be very time consuming. The reason for this is an exponential decreasing balancing process in the Delta-winding. This balancing process has a time constant of τ=L/R. The smaller the DC-resistance R of the winding, the longer is the duration of this process. On large generator transformers, this balancing process may for example take several minutes up to one hour. The vector group of generator transformers is usually ‘YNd’ with any phase shift (YNd1, YNd3, YNd5, YNd7, YNd9 and YNd11). The identifier ‘Y’ means that the primary-side topology of the transformer is a star topology, the identifier ‘N’ means that the transformer has a neutral point, the identifier ‘d’ means that the secondary-side topology of the transformer is a Delta-topology and the subsequent number indicates the factor for the calculation of the phase shift between the phases of the transformer legs by multiplying the number with an angle of 30°. For example, in an ‘YNd3’-configuration the phase shift is of 90°.
In existing solutions like high current testers, the measurement time is reduced by using measuring currents which are higher than the core saturation level. This is done because the main inductance L becomes smaller and thus the time constant τ becomes shorter, as the winding resistance R is constant at a given temperature.
These high current testers of the prior art are large and heavy devices utilizing massive conductors and connectors to supply 50 A or more to saturate the core. Moreover, on large power transformers, the saturation current can easily reach 100 A or more. Therefore, even such high current measurement devices cannot saturate the core in case they cannot supply the required current.
Thus, a common practice is to drive the measuring current simultaneously through the secondary-side and the primary side winding of the transformer to be tested using a series connection of them. The current in the primary side winding assists in saturating the transformer core, when measuring the secondary side winding. Because the turns ratio γ (γ=N1/N2) is usually greater or equal to 10 on generator transformers, wherein N1 denotes the primary side number of turns and N2 the secondary side number of turns, the current necessary to saturate the core is ten times or more smaller on the primary side than on the secondary side. The result of this practice is a decreased measuring time.